For many years, venturis have been used to measure fluid flow rates for both liquids and gases. Venturis are often used to measure fluid flow since they generally cause less permanent loss of fluid pressure as compared to other metering devices such as an orifice or nozzle. Considerable testing and development has been conducted with respect to using venturis to measure and/or control high flow rates of compressible fluids associated with jet engines, rocket motors and steam turbines. Venturis in which fluid travels at the speed of sound through the narrowest restriction of the venturis are often referred to as critical flow venturis. Such venturis have been used to provide accurate, simple and highly reliable flow meters. However, most critical flow venturis can only be used to measure or control discrete fluid flow rates within a limited range depending upon the dimensions of the narrowest restriction within the venturi and fluid temperature, pressure, and composition upstream from the respective venturi.
If fluid flowing in a closed conduit is accelerated through a geometrical restriction of appropriate dimensions relative to dimensions of the conduit upstream from the restriction, the speed of fluid flow at the narrowest portion of the restriction will reach the local speed of sound. When the local speed of sound is reached, the flow is said to be critical or sonic. Knowledge of the upstream thermodynamic state (such as knowledge of fluid pressure, temperature and composition) may be applied in conjunction with the known cross sectional area of the narrowest restriction in the venturi, to calculate the critical flow rate using the First and Second Laws of Thermodynamics, and state equations for fluid density and the sound speed in the respective fluid. One-dimensional and non-isentropic modeling errors can be correlated to the Reynolds number of the fluid flowing through the narrowest restriction. Critical flow conditions have been widely used to measure fluid flow rates for nearly half a decade using venturis with fixed cross-sectional flow areas at the narrowest restriction.
Current technology in critical flow metering of fluids typically uses a fixed geometry flow restriction that allows only one, discrete flow rate, the critical flow rate, to pass through the meter or critical flow venturi for a given condition of upstream fluid pressure, temperature and composition. Because of this critical flow rate limitation, most critical flow venturis have essentially no flow rate range. Thus, fixed geometry critical flow venturis are often connected in parallel metering conduit runs, with each run having a critical flow venturi capable of metering a single critical flow rate for a given upstream fluid pressure, temperature and composition. In such multiple metering conduit configurations, multiple discrete fluid flow rates may be achieved by valving combinations of individual metering runs open or closed.